1. Field of the Invention
This invention relates to micromechanical resonator devices and micromechanical devices utilizing same.
2. Background Art
Vibrating mechanical tank components, such as crystal and SAW resonators, are widely used for frequency selection in communication sub-systems because of their high quality factor (Q's in the tens of thousands) and exceptional stability against thermal variations and aging. In particular, the majority of heterodyning communication transceivers rely heavily upon the high-Q of SAW and bulk acoustic mechanical resonators to achieve adequate frequency selection in their RF and IF filtering stages and to realize the required low phase noise and stability in their local oscillators. In addition, discrete inductors and variable capacitors are used to properly tune and couple the front end sense and power amplifiers, and to implement widely tunable voltage-controlled oscillators. At present, the aforementioned resonators and discrete elements are off-chip components, and so must interface with integrated electronics at the board level, often consuming a sizable portion of the total sub-system area. In this respect, these devices pose an important bottleneck against the ultimate miniaturization and portability of wireless transceivers. For this reason, many research efforts have been focused upon strategies for either miniaturizing these components or eliminating the need for them altogether.
Recent demonstrations of micro-scale high-Q oscillators and mechanical bandpass filters with area dimensions on the order of 30 μm×20 μm now bring the first of the above strategies closer to reality. Such devices utilize high-Q, on-chip, micromechanical (abbreviated “μmechanical”) resonators constructed in polycrystalline silicon using IC-compatible surface micromachining fabrication techniques, and featuring Q's of over 80,000 under vacuum and center frequency temperature coefficients in the range of −10 ppm/° C. (several times less with nulling techniques). To date, resonators based on freely-supported, vibrating prismatic beams have achieved frequencies of up to 92 MHz. For use in many portable communications applications, however, higher frequencies must be achieved and are thus important to the success of this technology.
Much like the case for transistors, extending the frequency of mechanical resonators generally entails scaling of resonator dimensions. Some of the previous VHF demonstrations with clamped—clamped boundary conditions actually used submicron dimensions to avoid Q-limiting anchor losses. Unfortunately, smaller size often coincides with smaller power handling and increased susceptibility to environmental effects, such as contamination or thermal fluctuations. Although recently demonstrated free—free beam μmechanical resonators have been able to achieve frequencies up to 92 MHz with Q's around 8,000 while avoiding submicron dimensions, as shown in U.S. Pat. No. 6,249,073, whether or not they can maintain their size and Q at UHF frequencies has yet to be seen.